The present invention relates to tire inner tubes made from isobutylene-co-paramethylstyrene, elastomers, and blends thereof
Tire inner tubes are ordinarily made from butyl rubber cross-linked or vulcanized to form a tube having good heat stability and physical properties. However, high temperatures and extreme conditions of use cause repeated expansion and contraction of the tire inner tubes. As a result, inner tubes made from these polymers have a limited useful life. For example, in many developing nations having bad roads and tropical and sub-tropical conditions, inner tubes are still used within tires. Inner tubes comprised of isobutylene based rubbers, such as butyl rubber, and chlorobutyl rubber, and EPDM rubber, do not have the proper properties to survive severe conditions of overloading and high temperatures. The inner tubes degrade, lose air, and become sticky, adhering to the tire. The inner tube cannot be removed from the tire, the tire cannot be retreaded, and thus the tire must be scrapped. These deficiencies in current inner tube compositions lead to excessive waste of tire composition and added cost of replacing a tire.
Most present day inner tube compositions lack the superior heat aging properties needed to produce a reliable, improved air impermeable tire inner tube capable of performing in severe service applications such as high speed, bumpy roads and overloading situations, which all may cause rapid deformation.
Inner tubes comprising butyl rubber are superior in barrier properties to inner tubes formed from any other rubber. However, certain applications require improved heat resistance which is normally obtained by blending Ethylene-Propylene (EP) or EPDM rubber with butyl in inner tube compounds. However, blending of EP or EPDM rubbers increases the air permeability of the rubbers.
U.S. Pat. No. 5,698,640 teaches isobutylene-co-brominated paramethylstyrene (EXXPRO(trademark) ExxonMobil Chemical Company) bladders made from a low-bromine formulation. However, this low-bromine formulation does not possess proper cure properties needed for the production of inner tubes. The low bromine polymer will not develop the tensile and modulus strength needed for inner tubes. The cure system disclosed in U.S. Pat. No. 5,698,640 is considered to be too xe2x80x9cscorchyxe2x80x9d for inner tube processing.
U.S. Pat. No. 5,576,373 teaches the use of layered silicates in inner tubes while WO 9422680 includes a laundry list of rubbers that may be used for a variety of applications.
U.S. Pat. No. 5,650,454 discloses an elastomer composition comprising an isobutylene-paramethylstyrene copolymer rubber that may be used in inner tubes. The composition must contain an additive comprising a cross-linked fatty acid and a starch. The additive is said to increase tear resistance, but there is no suggestion that the compositions formed have enhanced thermal stability.
It has been known to use 1,6-hexamethylene-bis(sodium thiosulfate) (HTS, Flexsys Corp.) as a stabilizing agent, more specifically as a reversion inhibitor, in sulfur-vulcanized master batches of high diene rubber. See U.S. Pat. Nos. 4,417,012; 4,520,154; and 4,587,296 to Moniotte; and U.S. Pat. No. 5,508,354 to Talma et al. herein incorporated by reference for purposes of U.S. patent practice.
There is a need for an inner tube that will have enhanced thermal stability and physical properties under severe temperature and operating conditions.
It has been found that halogenated isobutylene-co-paramethylstyrene copolymer, preferably brominated isobutylene-co-paramethylstyrene copolymer, can be effectively used as a composition suitable for severe duties such as tire inner tubes.
Accordingly, in one aspect of the present disclosure, inner tubes comprising halogenated isobutylene-co-paramethylstyrene polymer, preferably brominated isobutylene-co-paramethylstyrene polymer (BIMS) exhibit improved heat resistance while retaining the superior barrier properties of butyl inner tubes.
In another aspect, the present invention provides an inner tube comprising a blend of a halogenated isobutylene-co-paramethylstyrene polymer, preferably brominated isobutylene-co-paramethylstyrene polymer, and a second isobutylene based rubber.
In another aspect, the present invention provides a composition that is heat resistant and thermostable and may be used in articles of manufacture that require such properties, such as high temperature hoses, gaskets, seals, and belts.
Tire inner tubes are generally defined as inflatable air containers or bladders that are usually positioned between a tire carcass and a tire rim or wheel. Tire inner tubes can be employed in a variety of tires, such as bicycle tires, automobile, and truck tires. Although most automobile and truck tires in the industrialized world are tubeless, many developing nations, such as India, China, Indonesia, and African nations still use tires with inner tubes. In these developing nations, high temperatures, bad roads and extreme conditions of use cause repeated expansion and contraction of the tire inner tubes. As a result, inner tubes made from these polymers have a limited useful life.
In the present invention, compositions comprising halogenated isobutylene-co-paramethylstyrene polymers, preferably brominated isobutylene-co-paramethylstyrene polymers and blends of a second isobutylene based rubber and BIMS, exhibit improved heat resistance while retaining the superior barrier properties of butyl rubber inner tubes. The inner tube may comprise halogenated isobutylene-co-paramethylstyrene polymer, preferably brominated isobutylene-co-paramethylstyrene polymer or a blend comprising a second isobutylene based rubber and BIMS. The inner tube comprising the BIMS alone or the blend comprising at least 15 parts per hundred rubber halogenated isobutylene-co-paramethylstyrene in one embodiment, at least 25 parts per hundred rubber halogenated isobutylene-co-paramethylstyrene in another embodiment, and at least 40 parts per hundred rubber halogenated isobutylene-co-paramethylstyrene in yet another embodiment. As previously stated, the halogenated isobutylene-co-paramethylstyrene polymers are preferably brominated isobutylene-co-paramethylstyrene polymers (BIMS). The term parts per hundred rubber or xe2x80x9cphrxe2x80x9d is well understood in the art.
Isobutylene based polymers, particularly halogenated isobutylene based polymers, and more particularly halogenated butyl rubber are the primary compositions of most tire liners, heat resistant tubes, and other commercially known products such as pharmaceutical ware. The term xe2x80x9cbutyl rubberxe2x80x9d as employed herein is intended to refer to a vulcanizable rubbery copolymer containing, by weight, from about 85% to about 99.5% combined isoolefin having from 4 to 8 carbon atoms, such as a copolymer of isobutylene/isoprene. Such copolymers and their preparation are well known to one skilled in the art. A preferred butyl rubber is shown in the following examples and is commercially available from ExxonMobil Chemical Company as Exxon(copyright) Butyl 268 rubber (hereinafter referred to as xe2x80x9cButyl 268xe2x80x9d rubber).
Brominated isobutylene-co-paramethylstyrene polymer is well known in the art. The preparation of C4-C7 isoolefin/paralkylstyrene copolymers is generally disclosed, for example, in U.S. Pat. No. 5,162,445 to Powers et al. and U.S. S.I.R H1475 by Newman et al. both of which are incorporated herein by reference for purposes of U.S. patent practice. BIMS with a relatively low level of bromine, at least 0.5 moles percent benzylic bromine relative to the total amount of copolymer present, is commercially available from ExxonMobil Chemical Company as EXXPRO(trademark) elastomer.
According to the one embodiment of the invention, the BIMS has a relatively low bromination level, from 0.1 to 3.0 mol % benzylic bromine (relative to the total amount of copolymer) in one embodiment, and 0.25 to 2.0 mol % in another embodiment, and 0.60 to 1.5 mol % benzylic bromine in yet another embodiment. Below this range, the crosslinking density obtained following vulcanization may be too low to be useful. Above this range, the polymer will be too tightly crosslinked using the same cure package to have useful properties, e.g. it will become stiff or brittle, or there may be unreacted benzylic bromine that can lead to instability. In a tire inner tube this will cause reduced life of the vulcanizate, and undesirably increased adhesion or a tendency to co-cure with halobutyl tire innerliners. Within the range in accordance with the invention, the bromine content, as well as the other polymer properties and the choice of cure package can be adjusted and optimized to obtain desired properties of the resulting vulcanizate.
In general, the bromination level of the BIMS is controlled by limiting the amount of bromine reactant using the procedures and techniques disclosed in U.S. Pat. No. 5,162,445 to Powers et al. and U.S. S.I.R. H1475 by Newman et al. mentioned above.
Halogenation of the base isobutylene-co-paramethylstyrene polymer is one method of functionalizing the polymer for future vulcanization and processing. An alternative preparation of a functionalized isobutylene-co-paramethylstyrene polymer is metallation of the isobutylene-co-paramethylstyrene polymer, as is illustrated by U.S. Pat. Nos. 5,670,581, 5,849,828, 5,821,308, and 5,840,810 to Frechet et al., which are incorporated herein by reference for purposes of U.S. patent practice.
In one embodiment, the BIMS comprises from 0.5 to 10 mol % para-alkylstyrene, from 1.0 to 8.0 mol % in another embodiment, and from 1.5 to 5.0 mol % para-alkylstyrene in another embodiment. In one embodiment the BIMS comprises from 0.1 to 3.0 mol % benzylic bromine, from 0.25 to 2.0 mol % in another embodiment, and 0.6 to 1.5 mol % benzylic bromine in yet another embodiment.
The BIMS used in the present application comprises about 3.75 mol % p-methylstyrene and about 1.2 mol % benzylic bromine (commercially available as EXXPRO(trademark) 3745 elastomer) or 2.5 mol % paramethylstyrene and about 0.75 mol % benzylic bromine (commercially available as EXXPRO(trademark) 7433).
For use in tire inner tubes and other severe duty applications, the BIMS generally has a weight average molecular weight from about 50,000 to about 700,000, from about 300,000 to about 550,000 in one embodiment. Mooney viscosities at 125xc2x0 C. from 30 to 60 are common, with about 45 in one embodiment. The particular Mooney viscosity of the BIMS will depend on the particular properties desired for the vulcanizate application.
A preferred cure package comprises vulcanizing agents and may further include conventional accelerators, retarders, and activators. Accelerators include, for example, fatty acids such as stearic acid and fatty acid metal salts, tetramethyl thiuram disulfide (TMTD) (commercially available from Flexsys Corp. of Akron, Ohio), bis (2,2xe2x80x2-benzothiazole disulfide) (also available from Flexsys Corp.). Suitable retarders include, but are not limited to, DHT4A2, magnesium aluminum hydroxy carbonate, commercially available from Mitsui Chemical Corp. The accelerators and retarders are used in the cure package from 0 to 12 phr in one embodiment, and from 4 to 10 phr in another embodiment, and in yet another embodiment at 7 phr.
The vulcanizing agent may be any suitable agent, typically sulfur, or zinc oxide, or both in combination. When halogenated isobutylene based rubbers and EXXPRO(trademark) elastomers are used as a blend, the vulcanizing agent is preferably zinc oxide.
The blend of the butyl rubber, BIMS and cure package may also be compounded with reinforcing fillers, such as carbon black (commercially available as N660), oils, napthenic or parraffinic, (preferably FLEXON(trademark) 641, a napthenic oil commercially available from ExxonMobil Chemical Company), waxes and other additives, such as anti-oxidants and anti-ozonants, conventionally used in the manufacture of tire inner tubes and other severe duty elastomers.
The polymer may be prepared using a one step mix, a two-step mix, an internal mixer, a two-roll mix or combinations thereof The inner tube composition is preferably prepared in an internal mixer using a two step mix. A preferred mix procedure comprises preparing a masterbatch of the polymer/polymers, carbon black, and oil. The batch is removed or dumped between 120xc2x0 C. and 160xc2x0 C. The master batch is then remixed with the curative package and dumped between 100xc2x0 C. to 120xc2x0 C.
The blend of BIMS/curative mixture and/or isobutylene based elastomer is then shaped or molded and cured using conventional shaping and vulcanization equipment. For example, tire inner tubes are preferably extruded and then press cured in a mold.